In recent years, along with a tendency for high brightness and whitening of a light-emitting device using a light-emitting element such as a light-emitting diode, such a light-emitting device has been used for illumination, backlight of various displays or large-sized liquid crystal TVs, etc. As a light-emitting device, for example, a light-emitting device having a lead frame mainly made of copper and on which a light-emitting element is mounted and a reflective frame so-called a reflector to surround a mounting portion of the lead frame on which a light-emitting element is mounted, has been known. In the case of such a light-emitting device, since the lead frame has a high heat conductivity, an excellent heat release property can be obtained with a relatively simple structure.
The reflector for a light-emitting device is made of, for example, a resin material in which a white inorganic pigment such as titania is dispersed and formed in an integrated structure to surround the mounting portion of the lead frame. However, the reflector made of the resin material is likely to be colored due to aged deterioration by light from a light-emitting element, and the reflectance may be lowered. Particularly, in a case where a light-emitting element which emits ultraviolet rays, near ultraviolet rays or blue light is used, the coloration or the deterioration of the reflectance due to such aged deterioration is remarkable.
In order to suppress the coloration or the deterioration of the reflectance due to the aged deterioration, it has been known, for example, to use a reflector made of a ceramic material. Since the ceramic material is an inorganic material, as compared to a resin material which is an organic material, the coloration or the deterioration of the reflectance due to aged deterioration is easily suppressed. However, since it is necessary to fire a ceramic material at a temperature exceeding 1,500° C., its productivity is not good. Further, in a case where a ceramic material is fired with a lead frame made of copper or the like, the lead frame may melt. Further, in a case of a reflector made of a ceramic material, a high reflectance cannot always be obtained.
For example, it has been known to form voids by air bubbles in a reflector made of a ceramic material in order to increase a reflectance. However, if the number of voids is large, each treating liquid used in production steps of a light-emitting device tends to penetrate into them, and as a result, the reliability of a light-emitting device may be low. Further, in a case where the inside of a reflector is filled with a sealing material to seal a light-emitting element, etc., water and vapor tend to penetrate into an interface between the reflector and the sealing material, and the reliability of the light-emitting device cannot be necessary enough.
Further, a reflector made of conventional borosilicate glass and a reflector made of a conventional borosilicate glass in which alumina, magnesia, barium sulfate or the like is incorporated are known. However, a specific composition, etc. of borosilicate glass are not necessarily clear, and properties such as reflectivity and the reliability of the borosilicate glass are not necessarily clear (e.g. Patent Document 1).
On the other hand, known is a substrate for mounting a light-emitting element, which is formed by forming a green sheet made of a glass ceramics composition containing a glass powder and a ceramic filler, laminating plural such green sheets, firing it at from 850 to 900° C. and then dividing it into individual species (Patent Document 2). In a case where a substrate for mounting a light-emitting element is formed by using such a green sheet, it is necessary to increase deflective strength in order to prevent defections such that the green sheet is broken or cracked at a time of dividing. That is why it is necessary to increase the sintering density, and when the glass ceramics composition is sintered, sintering is carried out so as to be dense to prevent voids from remaining (porosity is almost 0%). Further, silver conductors used as wiring conductors or conductive vias mainly have a role as a heat release structure. In addition, heat conductivity of the substrate itself is desired to be high. Here, if a sintered substrate has voids, the heat conductivity is remarkably lowered, which causes deterioration of the heat release property. That is, from a viewpoint of the strength and also from a viewpoint of the heat release property, after sintering the glass ceramics composition, the glass ceramics composition is sintered so as to prevent voids from remaining.
Further, in order to form a green sheet which is flexible and easy to handle, it is necessary to incorporate a large amount of a resin in a green sheet. Thus, it is necessary to heat decompose the resin in a firing step, and a heat treatment step at from 400 to 500° C. for about 5 hours is usually required.
Further, a low temperature firing substrate technique in which a green sheet is used is originally employed for simultaneously firing a silver conductive layer which is formed by printing or the like, and a firing temperature of at least 850° C. is required for sufficiently firing the silver conductive layer.